Xenon's anesthetic properties have been known to the medical profession for over 50 years (Cullen and Gross, 1951). However, despite some impressive displays of clinical efficacy in patients (Luttropp et al., 1994; Lynch et al., 2000), everyday use of xenon anesthesia has failed to materialize. This is largely associated with the significant cost involved in production of xenon through fractional distillation of liquid air, and hence the relatively small percentage of the total refined quantity of xenon available for anesthesia (Hanne Marx et al., 2001). Consequently, use of xenon is likely to be restricted to special areas where there is an appreciable cost-benefit advantage. One such area may be neonatal anesthesia, where xenon may lack harmful side effects seen with other commonly used neonatal anesthetics e.g. nitrous oxide (Layzer, 1978; Amos et al., 1982; Jevtovic-Todorovic et al., 1998).
It is well documented in the art that neonatal insults cause long lasting effects (Anand and Scalzo, 2000; Balduini et al., 2000; Jevtovic-Todorovic et al., 2003). Therefore it is sensible to adopt a degree of caution when using drugs in the neonate which could potentially alter neurodevelopment (such as alcohol, phencyclidine, ketamine, N2O, isoflurane, benzodiazepines, barbiturates and anticonvulsants (Olney et al., 2002d) by causing apoptotic neurodegeneration). This is especially true given that often only a single exposure is required, even at anesthetic doses (Ikonomidou et al., 2001; Young et al., 2003).
Normal Neurodevelopment
Normal neurodevelopment is a carefully regulated (Butler, 1999) sequence of events including proliferation, differentiation, migration and synaptogenesis. Glutamate is thought to have a role in all of these processes (Ikonomidou and Lechoslaw, 2002), for example high concentrations of glutamate at migration target zones suggests a role as a neuronal chemoattractant (Behar et al., 1999) along with the NMDA receptor used to detect it (Komuro and Rakie, 1993). The intriguing finding of specific NMDA receptor subtypes (e.g. NR2B and NR2D) in different anatomical regions may shed light on the precise nature of migration control (Behar et al., 1999). From work by the same group, it is also apparent that different species employ different mediators in migration control—currently either GABA (rats) or glutamate (mice) (Behar et al., 2001).
Synaptogenesis (the brain growth spurt) is a period of a rapid establishment of synapses, characterised by a high level of physiological cell death (up to 1% (Olney et al., 2002b)). This includes the formation of extensive corticothalamic and thalamocortical projections (Molar and Blakemore, 1995). Despite the immense complexity of inter-species embryology, it has been shown that comparisons can be made because milestones in neurodevelopment tend to occur in the same sequence (Clancy et al., 2001). This permits an extrapolation of the period of peak synaptogenic activity from the 7 day old rat pup (Olney et al., 2002a) to a 0-8 month old human being (Ikonomidou et al., 1999; Jevtovic-Todorovic et al., 2003). However, based on analysis of NMDA receptor subtypes, it is more probable that humans experience an extended period of synaptogenesis—from the beginning of the 3rd trimester of pregnancy to several years old (Dobbing and Sands, 1979; Jevtovic-Todorovic et al., 2003).
Apoptosis in the Developing Nervous System
Apoptosis, first formally described in 1972 (Kerr et al., 1972), is an essential feature of normal neurodevelopment in processes such as sculpturing, trimming, control of cell numbers and cellular disposal. It is characterised as “active cell death” comprising initiation, commitment and execution by dedicated cellular proteins (Sloviter, 2002). The crucial role of apoptosis is highlighted by the fact that genetic upregulation or downregulation of apoptosis results in a lethal genotype (Yoshida et al., 1998; Rinkenberger et al., 2000).
Control of physiological cell death (PCD) in the immature CNS is currently thought to be governed by the neurotrophic hypothesis—whereby neurones which fail to reach their survival promoting synaptic targets (Sherrard and Bower, 1998) initiate a specialised form of cell suicide secondary to withdrawal of environmental trophic support (Young et al., 1999) (via both neurotrophins and electrical stimulation) (Brenneman et al., 1990). Due to the complex divergent and convergent nature of the “survival pathway” many ligands and mechanisms are involved in maintaining neuronal survival. The cytosol and mitochondria of neurones field a balanced assortment of molecules which are either anti-apoptotic (e.g. Bcl-2 and cAMP response binding protein) or pro-apoptotic (e.g. Bad, Bax and the caspase family) which determine cell fate. Bcl-2 and its associated peptides are thought to be particularly important in the developing CNS (Yuan and Yanker, 2000), as evidenced by the high levels of expression in the neonate and the fact that experimental over-expression of Bcl-2 can both override lack of trophic support (Garcia et al., 1992), and even prevent PCD altogether (Martinou et al., 1994). A variant of Bcl-2 (Bcl-XL) may have a specialised role in maintaining developing neurones before they have found their synaptic targets (Motoyama et al., 1995).
Neurodegeneration in Neonates
In 1999, data were published showing that use of NMDA receptor antagonists in neonatal rats produced specific patterns of neurodegeneration (distinct from glial cells) (Ikonomidou et al., 1999). On electron microscopy, this neurodegeneration was identical to apoptotic cell death, and most evident in the laterodorsal thalamic nucleus, one of the areas of the brain implicated in learning and memory (Goen et al., 2002). This phenomenon has since been demonstrated in other brain regions with other drugs (Monti and Contestabile, 2000).
Later work done by Jevtovic-Todorovic et al. showed that neonatal rats are vulnerable to harmful side effects of anesthesia during the synaptogenic period. They demonstrated up to a 68-fold increase in the number of degenerating neurones above baseline in areas such as the laterodorsal and anteroventral thalamic nuclei (and to some extent layer II of the parietal cortex) after exposure to anesthetic agents (Jevtovic-Todorovic et al., 2003), which resulted in a functional neurological deficit in behaviour tests later in life. Specifically, the GABAergic anesthetic isoflurane (Gyulai et al., 2001), produced dose-dependent neurodegeneration in its own right, with synergistic neurodegeneration with the successive addition of midazolam (a double GABAergic cocktail) and then N2O (a triple cocktail) (Jevtovic-Todorovic et al., 2003). This process has been shown to occur with exposure to GABAergic agents in areas other than anesthesia, such as anticonvulsant therapy and maternal drug abuse in rats (Bittigau et al., 2002; Farber and Olney, 2003).
A clinical manifestation of this type of neurodegeneration is detected in 1 to 2 infants per 1000 livebirths as Fetal Alcohol Syndrome (FAS) (Moore and Persaud, 1998)—characterised by abnormal facial features, microencephaly and mental retardation (Olney et al., 2002c). It is thought that binge drinking by pregnant mothers produces very high levels of ethanol (a dual GABAergic agent and NMDA receptor antagonist (Farber and Olney, 2003)) in the fetal brain, which in turn triggers the type of neurodegeneration discussed above (Ikonomidou et al., 2000). It is worth noting that this mechanism of action closely resembles that of current anesthetic procedures.
The present invention seeks to provide an anesthetic agent suitable for use in the newborn that is safe, efficacious, and does not have any adverse effects on neurodevelopment. More specifically, the invention seeks to provide an anesthetic agent for neonatal subjects that is suitable for use in combination with other anesthetics know to adversely affect neurodevelopment. In particular, the invention seeks to provide anesthetic combinations for use in neonates which comprise an agent capable of preventing or alleviating the adverse effects of known anesthetic agents such as isoflurane and/or sevoflurane, and/or desflurane.